1. Field of the Invention
This invention relates to a method and an apparatus for electric current control in gas generators which generate a fluorine or fluoride gas.
2. Description of the Related Art
Conventionally, fluorine is produced by electrolysis of a molten salt containing a fluoride such as HF, as shown in the equation (1):F−→½F2+e−  (1) (fluorine generation reaction).
On that occasion, hydrogen is generated from the cathode, as shown by the equation (2):2H++2e−→H2  (2) (hydrogen generation reaction).
However, among the reactions shown above by the equations (1) and (2), the fluorine generation reaction, which occurs on the anode, is accompanied by very complicated side reactions, as shown by the equations (3) to (10):xC+F−→(Cx+F−)+e−  (3) (fluorine-carbon intercalation compound formation reaction)
The reaction shown by the equation (3) is a reaction proceeding within electrode carbon crystals, by which reaction the surface energy of the crystals increases and the wetting thereof with the electrolytic bath is improved and, further, the conductivity thereof as the electrode is improved as a result of hole conduction caused by hole creation within the crystals by drawing of π electron on carbon atom toward fluorine atoms.C+2F2→CF4  (4) (carbon tetrafluoride formation reaction)
The reaction represented by the equation (4) indicates that the fluorine gas generated by electrolysis reacts with carbon atoms electrode surface to generate the carbon tetrafluoride gas. This gas, when it enters a fluorine-containing gas, in particular the fluorine gas, becomes an impurity and reduces the purity of the fluorine gas. This gas is close in such properties as boiling point to the fluorine gas and therefore is difficult to eliminate from the fluorine gas. Thus, the use of a carbon anode hardly allowing this reaction to occur is preferred from the high purity gas generation viewpoint.2H2O→O2+4H++4e−  (5) (oxygen generation reaction)xC+½O2→CxO  (6) (graphite oxide formation reaction)2xC+yF2→(CF)x  (7) (graphite fluoride generation reaction)
The equations (5) to (7) indicate a series of reactions. When water, which is lower in discharge potential than HF, is present in the electrolytic bath, water is electrolyzed according to the equation (5) before HF. The oxygen generated by this electrolytic reaction reacts with the electrode carbon to form graphite oxide according to the equation (6). This compound is unstable and the fluorine generated according to the equation (1) readily substitutes for the oxygen of this compound to generate graphite fluoride, as shown by the equation (7).
Graphite fluoride is very low in surface energy and, when graphite fluoride is formed on the electrode surface, that portion cannot come into contact with the electrolytic bath, causing polarization, which inhibits the progress of the electrolytic reaction. When the coverage of graphite fluoride, which is very low in surface energy, as mentioned above, exceeds 20% relative to the electrode surface area, the electrode surface will not be wetted with the electrolytic bath at all but the so-called “anode effect” condition will result. More specifically, the electrode cannot come into contact with the electrolytic bath, so that the resistance of the electrode surface becomes infinite and the path of the electrolytic current is thus barred, with the result that the electrolytic potential rapidly increases and a state arises in which electrolysis is no more possible at all.
This reaction tends to occur when the water content is high in the electrolytic bath, for example just after preparation of the electrolytic bath or just after starting of feeding of hydrogen fluoride as the raw material. When the increase in the current to be applied to the effective electrode surface area is excessive in electrolytic current application, too, these reactions tend to occur.
As the HF in the electrolytic bath is consumed, the HF concentration in the electrolytic bath comprising KF·xHF lowers and, when x becomes lower than 1.8, the ice point rises to 100° C. or above and the electrolytic bath precipitates out on the anode and cathode, respectively, at a controlled temperature of 90° C. to 100° C. under the operation conditions of the electrolyzer; in many cases, it precipitates out on the cathode (cylinder or nickel) rather than on the anode where graphite fluoride is formed according to the equation (7). When this phenomenon occurs, the bath voltage increases due to an increase in cathode resistance. This increase in bath voltage is a problem that can be solved by adjusting the HF concentration in the electrolytic bath to a predetermined level. However, once the melting point of the bath has risen and solidification has occurred, it is difficult to melt again the bath that has solidified in the electrolyzer. Therefore, once such a phenomenon has occurred, a much longer time is required for adjusting the HF concentration in a solidified portion as compared with HF concentration adjustment in the ordinary electrolytic bath that is in a molten state.Fe2+→+Fe3++e−  (8) (oxidation reaction of iron ions eluted)Ni2+→+Ni4++2e−  (9) (oxidation reaction of nickel ions eluted)
As shown by the equations (8) and (9), the iron and/or nickel ions electrochemically eluted from the structural materials of the electrolyzer are further oxidized on the anode to give Fe3+ or Ni4+. If the fluorides of these ions are present in the bath, they form complexes with KF. These complexes adhere to the anode in the manner of electrophoresis during electrolysis. These insulating deposits cause polarization on the anode. The phenomenon occurring during operation includes fluctuations and/or a slow rise in bath voltage. Further, when the contents of these impurities in the electrolytic bath increase, the viscosity of the electrolytic bath increases and splash entrainment tends to occur readily. When splash entrainment occurs, the electrolytic bath composition fluctuates with the lapse of time, possibly causing choking in piping portions and/or causing fluctuations in pressure in the electrolyzer.½F2+½H2→HF  (10) (reduction reaction of H2 and F2)
The reaction according to the equation (10) occurs when fluorine gas and hydrogen gas mix with each other. When this reaction occurs in the electrolytic bath, raw material recovery results, and the current efficiency in the fluorine generation reaction lowers. In any case, this is a reaction unfavorable for the maintenance of the main reaction in the electrolysis.
The reactions according to the above equations (1) to (10) except for the equation (2) occur on the anode. On the anode surface where such competitive reactions proceed, the surface conditions, inclusive of gas desorption and adsorption, are always changing, and this results in fluctuations in bath voltage relative to the current applied. Under such circumstances, a method of current application as resulting from due consideration of these reactions should be carried out so that fluorine may be generated smoothly with a current efficiency of 95% or higher even when use is made of a bath conditioned to sufficiently remove H2O in the bath.
In the case of industrial electrolyzers in ordinary use, the operation conditions are manually controlled, and watchmen adjust the operation conditions after observation by them of some or other noticeable abnormality, such as an abnormality in electrolytic voltage. Thus, they can operate only allopathically. Under the existing circumstances, when the electrolysis condition is found worsened, they lower the output repeatedly and, finally, they stop the electrolysis for repairing. At the time of stopping the electrolysis, the electrode is also found damaged in many instances, hence electrode replacement becomes essential. When, on that occasion, the suspension period and the manpower required for repairing and other factors are taken into consideration, this repair work costs very much. Considering these together, it is necessary to always monitor the electrolyzer condition automatically by means of a control system, not by watchmen, so that the electrolyzer may be operated stably while preventing any factors from inhibiting the electrolysis in accordance with the electrolyzer condition.
Under such circumstances, automatic operation has been attempted, for example, by on/off operations, depending on the bath liquid level, of the current supply means placed under the control of signals from a bath liquid level sensor provided within the electrolyzer so that the electrolysis conditions may be controlled and the liquid level may be maintained at a constant level (cf. e.g. JP Kohyo H09-505853).
However, as for the method described in the above-cited patent document, the current situation is that operators on site monitor the state of electrolysis and control the electrolysis conditions according to changes therein until it becomes possible to effect stable gas generation.
It is an object of the present invention, which has been made in view of the problems discussed above, to provide a method and an apparatus for current control in gas generators capable of generating a fluorine or fluoride gas by which method and apparatus the electrolysis can be maintained in an optimum state and stable operation is made possible without requiring manpower.